The 6V LEDs I got (not too expensive, $10-$15 per pair) are way brighter than stock. They might even be brighter than a modern car's OEM tail lights, but I haven't taken a lightmeter to them yet to compare.
If I were to do it again, I would get the pineapple style in red (like my turn signals) for tail lights rather than the brighter whites I bought. It would be a little cheaper, and I think the end result would be better. The whites are so bright, you get some bleed through around the edges making the tail lamp red with a whitish overtone. The downside would be that the license plate would be lit red. Maybe whites on the lower socket and reds higher would be the optimum.
Honestly, the LEDs are so, so much better than stock, or any other filiment bulb I've tried, that I think everyone should give it a try.
I have yet to swap dash lights for LEDs, so I can't give any opinion on that, but once the current bulbs go out, I will swap them. It might be hard to find a 6v LED that isn't too bright though.
